The synchronization technology has been widely used by carrying out clock synchronization, time synchronization or both on all apparatuses included in a system or on only necessary portions of the apparatuses by using wired and wireless lines in all industrial fields irrespective of the accuracy of synchronization, along with the recent digitization of information, automation and association among a variety of apparatuses, and refinement.
For example, the technology is used for synchronization between portable base stations connected to a mobile backhaul entrance line (a transmission line) and for detecting a location of failure in an electrical distribution network. With this synchronization technology, a GPS (Global Positioning System) is used mainly as a reference clock source and a reference time source. A synchronization technique that uses the GPS is stable and useful irrespective of the synchronization system being provided on the land, in the sky, and on the ocean when a reception antenna is open to the sky. However, due to some factors including the setting position of the reception antenna (e.g., not being open to the sky), weather, and solar flare, the reception antenna is impeded from receiving the signal sent from the GPS satellite, so the synchronization accuracy needed for the system is not satisfied.
Consequently, in the telecom field, the authentication and security field, and the electrical distribution network field, as a synchronization technique via a communication line for replacing the synchronization technique that uses the GPS, ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G.8264 and the like are specified for clock synchronization; and NTP (Network Time Protocol) is specified in RFC (Request For Comments) in time synchronization (see Non Patent Literatures 1 and 2). Further, studies by standardization organizations (ITU-T, IEEE, etc.) on clock and time synchronization are proceeding with IEEE1588, IEEE C37.238-2011, ITU-T G.8265.1, and the like (see Non Patent Literatures 3 to 5).